What is NOT generally regarded as a primary environmental pollutant originating in human activity? Select one: a. Anthropogenic emissions of sulfur dioxide and aerosols, b. Global warming due to build-up of greenhouse gases in the troposphere c. Problems associated with using sewage sludge from municipal wastewater treatment facilities as a fertilizer on agricultural soils, d. Methylation of mercury in river/lake sediments, e. Release of methyl isocyanate from pesticide manufacturing plants